A male & female Swimming Crab (sometimes called Harbour crab), Liocarcinus depurator showing clasping behaviour. Many crabs only mate when the female is soft after casting her shell. The male can detect that this is to occur well in advance and will carry the female around for several days to prevent other males claiming her. Immediatly after she moults, the female is turned over so that the abdomens are touching when copulation takes place. See next picture for mating position.
North Sea - St. Abbs Marine Reserve